Apparatus and methods for z-axis control and collision detection and recovery for waterjet cutting systems

ABSTRACT

This invention relates to apparatus and methods for z-axis control and collision detection and recovery for waterjet and abrasive-jet cutting systems. In one embodiment, an apparatus includes a linear rail, a slide member coupleable to a cutting head and slideably coupled to the linear rail, at least one actuator having a first end coupled to the slide member and a second end fixed with respect to the linear rail, a position sensor, and a controller. The actuator provides an adjustable support force that supports the weight of the cutting head, allowing the cutting head to be controllably positioned at a desired height above the workpiece. The actuator may include a pneumatic cylinder, or alternately, a linear motor. In another aspect, an apparatus includes a first mount member coupleable to a controllably positionable mounting surface of the waterjet cutting system, a second mount member coupleable to the cutting head and disengageably coupled to the first mount member, and a sensing circuit having a plurality of first conductive elements disposed on the first mount member and a plurality of second conductive elements disposed on the second mount member. In the event of a collision between the cutting head and an obstruction, the second mount member disengages from the first mount member to prevent breakage of the cutting head. Following the collision, the second mount member is quickly and easily re-engaged with the first mount member without time-consuming re-calibration. In one embodiment, re-engagement of the second end first mount members is automatically performed by a biasing member.

TECHNICAL FIELD

This invention relates to apparatus and methods for z-axis control andcollision detection and recovery for waterjet and abrasive-jet cuttingsystems.

BACKGROUND OF THE INVENTION

Waterjet and abrasive-jet cutting systems are used for cutting a widevariety of materials. In a typical waterjet cutting system, ahigh-pressure fluid (e.g., water) flows through a cutting head having acutting nozzle that directs a cutting jet onto a workpiece. The cuttingnozzle may include a mixing tube for introducing an abrasive into thehigh-pressure cutting jet to form an abrasive cutting jet. The cuttingnozzle may then be controllably moved across the workpiece to cut theworkpiece into the desired shape. After the cutting jet (or abrasivecutting jet) passes through the workpiece, the energy of the cutting jetis dissipated and the fluid is collected in a catcher tank for disposal.Waterjet and abrasive jet cutting systems of this type are shown anddescribed, for example, in U.S. Pat. No. 5,643,058 issued to Erichsen etal. and assigned to Flow International Corp. of Kent, Wash., whichpatent is incorporated herein by reference. The '058 patent correspondsto Flow International's Paser 3 abrasive cutting systems.

FIG. 1 is an isometric view of a waterjet cutting system 10 inaccordance with the prior art. The waterjet cutting system 10 includes acutting head 20 coupled to a mount assembly 30. The mount assembly 30 iscontrollably driven by a control gantry 40 having a drive assembly 42that controllably positions the cutting head 20 throughout an x-y planethat is substantially parallel to a surface 14 of a workpiece 12.Typically, the drive assembly 42 may include a pair of ball-screw drivesoriented along the x and y axes and a pair of electric drive motors.Alternately, the drive assembly 42 may include a five axis motionsystem. Two-axis and five-axis control gantries arecommercially-available as the Bengal 4x4 cutting systems from lowInternational of Kent, Washington.

FIG. 2 is a partial-elevational side view of the cutting head 20 and themount assembly 30 of the waterjet cutting system 100 of FIG. 1. Thecutting head 20 includes a high-pressure fluid inlet 22 coupled to ahigh-pressure fluid source 50, such as a high-pressure or ultra-highpressure pump, by a high-pressure line 23. In this embodiment, thecutting head 20 includes a nozzle body 24 and a mixing tube 26terminating in a jet exit port 28. Although the term “mixing tube” iscommonly used to refer to that portion of the cutting head of anabrasive jet cutting system in which abrasive is mixed with ahigh-pressure fluid jet to form an abrasive cutting jet, in thefollowing discussion, “mixing tube” is used to refer to that portion ofthe cutting head 20 that is closest to the workpiece 12, regardless ofwhether the waterjet cutting system uses an abrasive or non-abrasivecutting jet.

The mount assembly 30 includes a mounting arm 32 having a mountingaperture 34 disposed therethrough. The mounting arm 32 is coupled to alower portion 44 of the control gantry 40. The nozzle body 24 of thecutting head 20 is secured within the mounting aperture 34 of themounting arm 32.

In operation, high-pressure fluid from the high-pressure fluid source 50enters the high-pressure fluid inlet 22, travels through the nozzle body24 and mixing tube 26, and exits from the jet exit port 28 toward theworkpiece 12 as a cutting jet 16. The cutting jet 16 pierces theworkpiece 12 and performs the desired cutting. Using the control gantry40, the cutting head 20 is traversed across the workpiece 12 in thedesired direction or pattern.

To maximize the efficiency and quality of the cut, a standoff distance d(FIG. 2) between the jet exit port 28 of the mixing tube 26 and thesurface 14 of the workpiece 12 must be carefully controlled. If thestandoff distance d is to close, the mixing tube 26 can plug duringpiercing, causing system shutdown and possibly a damaged workpiece 12.If the distance is too far, the quality and accuracy of the cut suffers.

The mixing tube at 26 is typically fabricated of specially formulatedwear-resistant carbides to reduce wear. Particularly for abrasivecutting systems, the mixing tube 26 suffers extreme wear due to itsconstant contact with high velocity abrasives. Thus, mixing tubes are arelatively expensive component of the cutting head 20. The speciallyformulated carbides are also quite brittle, and can easily break if themixing tube 26 collides with an obstruction during operation of thecutting system 10, such as fixturing or cut-out portions of theworkpiece 12 which have been kicked up during the cutting operation.Accidental breakage of the mixing tube 26 increases operational costsand downtime of the cutting system 10.

Current collision sensors use a ring sensor disposed about the mixingtube 26 which slides along or slightly above the surface 14 of theworkpiece 12. The ring sensor indicates the relative height of theworkpiece. A motorized ball-screw drives the cutting head up and down tomaintain the required standoff distance. When the ring collides with akicked-up part or other obstruction, a detector detects the collisionand sends a stop signal to the control gantry to stop the movement ofthe mixing tube in an attempt to avoid the collision.

A fundamental problem with such collision sensors is that they must havea large enough “safety buffer” between the sensor and a mixing tube toallow the control gantry enough time to stop without damaging the mixingtube. Due to the size and speed of modem cutting systems, the task ofstopping the control gantry quickly to avoid a collision is quitedifficult. Another problem is that any shifting of the componentsrequires a lengthy re-calibration routine to insure proper standoffdistance d. A serious collision can ruin the ring sensor.

One approach has been to simply make the ring larger the allow tocontrol gantry more room to stop. This approach, however, prevents thecutting jet 16 from cutting near obstructions and fixtures commonlyfound around the edges of the workpiece 12, thereby wasting material.Enlarging the ring also increases the occurrence of erroneous collisionsignals which results in unnecessary downtime of the cutting system.Finally, existing ring sensor devices are expensive and are not robustin detecting surface height or collisions when operating the controlgantry at high-speed or under dirty conditions.

SUMMARY OF THE INVENTION

This invention relates to apparatus and methods for z-axis control andcollision detection and recovery for waterjet and abrasive-jet cuttingsystems. In one aspect of the invention, an apparatus includes a linearrail, a slide member coupleable to the cutting head and slideablycoupled to the linear rail, at least one actuator having a first endcoupled to the slide member and a second end fixed with respect to thelinear rail, a position sensor coupled to the slide member, and acontroller. The actuator provides an adjustable support force thatsupports the weight of the cutting head, allowing the cutting head to becontrollably positioned at a desired height above the workpiece. Theactuator may include a pneumatic cylinder, or alternately, a linearmotor.

In another aspect, an apparatus according to the invention includes afirst mount member coupleable to a controllably positionable mountingsurface of the waterjet cutting system, a second mount member coupleableto the cutting head and disengageably coupled to the first mount member,and a sensing circuit having a plurality of first conductive elementsdisposed on the first mount member and a plurality of second conductiveelements disposed on the second mount member. In the event of acollision between the cutting head and an obstruction, the second mountmember disengages from the first mount member to prevent breakage of thecutting head. Following the collision, the second mount member isquickly and easily re-engaged with the first mount member withouttime-consuming re-calibration. In one embodiment, re-engagement of thesecond and first mount members is automatically performed by a biasingmember.

In another aspect, a method of controlling a height of a cutting head ofa waterjet cutting system over a surface of a workpiece includescoupling a first end of a contact member to the cutting head, engaging asecond end of the contact member with the surface of the workpiece,providing an adjustably controllable support force to support a weightof the cutting head, and slightly reducing the support force to slightlydownwardly bias the contact member into engagement with the surface ofthe workpiece. The position control method advantageously provides asimple height measurement system and also allows for automaticadjustment for changes in friction or weight of various components ofthe waterjet cutting system.

BRIEF DESCRIPTIVE DRAWINGS

FIG. 1 is an isometric view of a waterjet cutting system in accordancewith the prior art.

FIG. 2 is a Me partial-elevational view of a cutting head and a mountassembly of the waterjet, cutting system of FIG. 1.

FIG. 3 is a front isometric view of a waterjet cutting system inaccordance with an embodiment of the invention.

FIG. 4 is a partial-sectional side view of a cutting head and adisengageable mount assembly of the waterjet cutting system of FIG. 3.

FIG. 5 is an exploded isometric view of the disengageable mount assemblyof FIG. 4.

FIG. 6 is a schematic view of a collision sensing circuit of thedisengageable mount assembly of FIG. 5.

FIG. 7 is a partially-exploded back isometric view of the waterjetcutting system of FIG. 3

FIG. 8 is a flowchart representation of a calibration routine of az-axis control system in accordance with an embodiment of the invention.

FIG. 9 is a flowchart representation of a biased-following routine of az-axis control system in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure is directed toward apparatus and methods forz-axis control and collision detection and recovery of cutting heads ofwaterjet cutting systems. Specific details of certain embodiments of theinvention are set forth in the following description, and in FIGS. 3-9to provide a thorough understanding of such embodiments. A person ofordinary skill in the art, however, will understand that the resentinvention may have additional embodiments, and that the invention may bepracticed without several of the details described in the followingdescription.

FIG. 3 is an isometric view of a waterjet cutting system 100 inaccordance with an embodiment of the invention. The waterjet cuttingsystem 100 includes a cutting head 120 coupled to a disengageable (or“breakaway”) mounting assembly 160. In the event of a collision, thedisengageable mounting assembly 160 advantageously disengages (or“breaks”) to prevent breakage of the mixing tube 26 or other cuttinghead components. After the collision occurs and the waterjet cuttingsystem 100 has been stopped, the disengageable mounting assembly 160 maybe easily re-engaged and the cutting operation continued without lengthyre-calibration procedures.

The waterjet cutting system 100 also includes a high-pressure fluidsource 50 fluidly coupled to the cutting head 120 by a coiledhigh-pressure line 123. The disengageable mounting system 160 isattached to a mounting arm 132, the mounting arm being coupled to acontrol gantry 40 as described above. The high-pressure fluid source 50may, for example, be a high-pressure or ultra-high pressure pump, suchas the commercially-available Husky pump models available from FlowInternational of Kent, Wash.

FIG. 4 is a partial-sectional side view of the cutting head 120 and thedisengageable mount assembly 160 of the waterjet cutting system 100 ofFIG. 3. FIG. 5 is an exploded isometric view of the disengageable mountassembly 160 of FIG. 4. As shown in FIG. 4, the cutting head 120includes a high-pressure fluid inlet 22 coupled to the coiledhigh-pressure line 123, a nozzle body 24 and a mixing tube 26. Themixing tube 26 includes a jet exit port 28 out of which a cutting jet 16emanates toward a workpiece 12. A collision shield 127 is disposed aboutthe mixing tube 26 to shield the mixing tube 26 from collisions. Thecollision shield 127 includes a wear ring 129. In some modes ofoperation of the waterjet cutting system 100, as described more fullybelow, the wear ring 129 engages a surface 14 of the workpiece 12, whilein other modes of operation the wear ring 129 is positioned slightlyabove the surface 14. The wear ring 129 may be formed of the samematerial as the collision shield 127, or alternately, may be formed of alow-friction material, such as, for example, Teflon®. The collisionshield 127 has a length I that is sized to provide a constant, desiredstandoff distance d between the jet exit port 28 and the surface 14.

The disengageable mounting assembly 160 includes a retainer 162 attachedto an upper surface 133 of the mounting arm 132. The mounting arm 132has an enlarged mounting aperture 134 disposed therethrough. Theretainer 162 includes a seating aperture 164 that is aligned with theenlarged mounting aperture 134 of the mounting arm 132. As best seen inFIG. 5, the retainer 162 further includes three pin cavities 166disposed about the circumference of the seating aperture 164. Each pincavity 166 has a pair of rounded pockets 168 disposed on opposite sidesof each cavity. An electrically-conductive strike pad 170 is positionedat the bottom of each rounded pocket 168. Similarly, anelectrically-conductive ball 172 is positioned within each roundedpocket 168 in contact with the associated strike pad 170.

A clamping collar 174 is attached to the nozzle body 24 of the cuttinghead 120 and is partially disposed within the seating aperture 164.Three conductive pins 176 project from the clamping collar 174. With theclamping collar 174 seated in the seating aperture 164, the conductivepins 176 projecting to the pin cavities 166 and contact the conductiveballs 172. The disengageable mounting assembly 160 also includes aseating force spring 178 disposed about the nozzle body 24 and engagedagainst a lower surface 135 of the mounting arm 132. A tensioner 179 isengaged onto the nozzle body 24 (e.g., threadedly engaged) and partiallycompresses the seating force spring 178. A collision sensing circuit 180is formed on the retainer 162, as described more fully below.

FIG. 6 is a schematic view of the collision sensing circuit 180 of thedisengageable mount assembly 160 of FIG. 5. The collision sensingcircuit 180 includes a plurality of conductive elements 182 coupled tothe strike pads 170 and to resistors 184 in parallel fashion. A voltagesource 186 is electrically coupled to the resistors 184. The strike pads170 are electrical contact with the conductive balls 172 which arecoupled by additional conductive elements 182 to ground 188. Eachresistor 184, strike pads 170, and conductive ball 172 form a branch ofthe parallel circuit. Secondary conductive elements 189 are electricallycoupled to a collision controller 190 and to the conductive elements 182between the resistors 24 and the strike pads 170. The collisioncontroller 190 transmits a first collision detection signal 192 to thehigh-pressure fluid source 50. The collision controller 190 alsotransmits of second collision detection signal 194 to the control gantry40 and a third collision detection signal 196 to a z-axis controlassembly 200, described more fully below.

The disengageable mounting assembly embodiment 160 shown in FIGS. 5 and6 is known as a Kelvin clamp. Kelvin clamps have been employed in touchprobes and other precision instrumentation, such as the coordinatemeasurement machines (CMM's) sold by Renishaw PLC of Gloucestershire,UK, as shown and described at www.renishaw.uk.com.

In operation, the disengageable mount assembly 160 prevents breakage ofthe mixing tube 26 by disengaging in the event of collision. As thecontrol gantry 40 moves the cutting head 120 in the x-y planesubstantially parallel to the surface 14 of the workpiece 12, the wearring 129 moves across the surface 14. In this embodiment, the collisionshield 127 is disposed about the mixing tube 26. When the collisionshield 127 strikes an obstruction, the force of the collision exerts atorque on the nozzle body 24 of the cutting head 120. The nozzle body 24begins to swing within the enlarged mounting aperture 134 of themounting arm 132, causing the clamping collar 174 to rotate within theseating aperture 164. The collision force required to pivot the nozzlebody 24 is determined by the amount of compression force into seatingforce spring 178, which is adjusted by adjusting the position of thetensioner 179.

As the clamping collar 174 rotates, one or more of the conductive pins176 become disengaged from the associated conductive balls 172, therebybreaking the circuit in one or more of the branches of the collisionsensing circuit 180. The collision controller 190 monitors the branchesof the collision sensing circuit 180 via the second conductive leads189, and detects the occurrence of the collision. The collisioncontroller 190 then transmits the first collision detection signal 192to the high-pressure fluid source 50 to shut off the flow ofhigh-pressure fluid through the cutting head 160. The collisioncontroller 190 also transmits the second collision detection signal 194to the control gantry 40 to stop movement of the cutting head 160.Finally, the collision controller 190 transmits the third collisiondetection signal 196 to the z-axis control system 200. Alternately, foran abrasive jet cutting systems, the collision controller 190 may alsotransmit a fourth collision detection signal to shutoff a flow ofabrasive to the cutting head 120.

After the waterjet cutting system 100 has been shut down by thecollision controller 190, the collision shield 127 is disengaged fromthe obstruction, and the disengageable mount assembly 160 is simplyre-engaged by re-seating the clamping collar 174 within the seatingaperture 164, and re-seating the conductive pins 176 within the pincavities 166. In this embodiment, the clamping collar 174 isautomatically re-seated within the seating aperture 164 by the force ofthe seating force spring 178. In alternate embodiments, the clampingcollar 174 may be manually re-seated within the seating aperture 164.After the conductive pins 176 have been re-seated, the branches of thesensing circuit 180 are re-established. The cutting head 120 may berepositioned by the control gantry 40, and a cutting operation may bequickly and easily resumed.

The disengageable mount assembly 160 advantageously prevents breakage ofthe mixing tube 26 and other components of the cutting head 120 in theevent of a collision. When a collision occurs, the cutting head 120simply pivots out of the way. At the same time, collision detectionsignals are generated which cause the various subsystems to stopautomatically. The disengageable mount assembly 160 allows the cuttinghead 120 to be returned to its pre-collision state with excellentrepeatability, preserving the machines calibration and allowing the userto resume cutting without any re-homing operations. Following acollision, the mount assembly 160 may be quickly re-engaged and thecutting operation resumed without re-calibration or other time-consumingprocedures.

One may note that although the disengageable mount assembly 160 has beenshown in the figures and described in the foregoing discussion as beinga Kelvin clamp, other disengageable mount assemblies are conceivablewhich may perform the function of pivoting the cutting head 120 out ofthe way in the event of a collision. Thus, while prior art collisionsensing systems focused on attempting to avoid a collision, theapparatus and method of the present invention acknowledges that acollision may be unavoidable, and accommodates the collision by means ofthe disengageable mount assembly.

FIG. 7 is a partially-exploded back isometric view of the waterjetcutting system 100 of FIG. 3. As shown in this view, the waterjetcutting system 100 includes a z-axis control system 200 disposed withina housing section 202. A back plate 204 is coupled to a pair of guideblocks 206 to enclose a backside of the housing section 202, and iscoupled to the control gantry 40. Thus, the z-axis control system 200 iscontrollably positioned by the control gantry 40 along with the cuttinghead 120.

The z-axis control system 200 further includes a pair of air cylinders208, each air cylinder having a first end 210 fixedly attached to thehousing section 202 and a second end 212 attached to a slide member 214.The mounting arm 132 is attached to the slide member 214. A linear rail216 is coupled to the slide member 214 and is disposed between the aircylinders 208. The linear rail 216 slideably engages the pair of guideblocks 206. An air brake 218 is attached to the slide member 214 andslideably engages the linear rail 216. The air cylinders 208 and the airbrake 218 are fluidly coupled to a high-pressure air source 220. Anfirst air control valve 222 controls to flow from the high-pressure airsource 220 to the air cylinders 208, and a second air control valve 223controls airflow to the air brake 218. The air brake 218 is preferably a“pressure to release” pneumatic brake that keeps the slide member 214 inposition and prevents the slide member 214 (and cutting head 120) fromfalling in the event of a loss of air pressure.

A position sensor 224 is attached to the slide member 214 between thesecond ends 212 of the air cylinders 208. In this embodiment, theposition sensor 224 includes a cable 226 attached to the uppermost guideblock 206. One commercially-available position sensor suitable for thispurpose, for example, is the LX-PA-15 String Potentiometer sold byUnimeasure, Inc. of Corvallis, Oreg. A z-axis controller 230 iselectrically coupled to the position sensor 224, to the first and secondair control valves 222, 223, and to the collision controller 190.

In operation, the z-axis control system 200 supports the weight of thecutting head 120, and rapidly raises and lowers the cutting head 120 bycontrolling the air pressure within the air cylinders 208. Thus, the aircylinders 208 provide a constant upward bias force that supports theweight of the cutting head 120, reducing the tracing force of thecollision shield 127 on the workpiece 12. If a collision is detected bythe collision controller 190, the collision controller 190 transmits thethird collision detection signal 196 to the z-axis controller 230. Thez-axis controller 230 transmits a brake control signal 231 to the secondair control valve 223, thereby releasing the air brake 218, and alsotransmits an air control signal 232 to the first air control valve 222,increasing the air pressure within the air cylinders 208 and raising theslide member 214. One may note that the functions of the z-axiscontroller 230 and the collision controller 190 may be integrated into asingle controller.

As the slide member 214 moves upwardly, the cable 226 is pulled out ofthe position sensor 224. The position sensor 224 determines the amountcable 226 drawn out by the movement of the slide member 214 andtransmits a position signal 228 to the z-axis controller 230. Inresponse to the position signal 228, the z-axis controller 230 transmitsan air control signal 232 to the air control valve 222 to raise or lowerabove air pressure within the air cylinders 208.

It is understood that the actuation device of the z-axis control system200 may be varied from the particular embodiment shown in FIG. 7 anddescribed above. For example, rather than a pair of air cylinders 208, asingle air cylinder may be employed. Alternately, the one or more aircylinders 208 may be replaced by linear motors. Commercially-availablelinear motors suitable for this purpose include, for example, those soldby Trilogy Systems of Webster, Tex. Generally, however, the aircylinders 208 are less expensive than alternate actuation devices.Commercially-available air cylinders suitable for this purpose include,for example, the Airpel® 16 m Air Cylinders sold by the AirportCorporation of Norwalk, Conn.

One advantage of the z-axis control system 200 is that it allows aunique mode of operation of the waterjet cutting system 100, referred toherein as “biased following.” Using the biased following method, thecutting head 120 is engaged with the surface 14 of the workpiece 12. Theheight of the workpiece 12 is therefore measurable simply by measuringthe position of the cutting head 120. Without the z-axis control system200, however, the relatively large weight of the cutting head 120 wouldcause undue and acceptable loading on the workpiece 12, preventing themethod of biased following from being used. The z-axis control system200 advantageously provides a constant upward bias force thataccommodates some or all of the way to the cutting head 120, therebygreatly reducing or eliminating the tracing force on the workpiece 12,allowing the method of biased following to be successfully used.

Another advantage of the z-axis control system 200 is that the cuttinghead 120 may be raised rapidly. Prior art ball-screw drive systemstypically are capable of raising or lowering the cutting head at a rateof approximately 40 cm/min. Using linear actuation devices, the z-axiscontrol system 200 is capable of raising or lowering the cutting head ata rate of approximately 40 cm/sec. Thus, the inventive z-axis controlsystem is approximately 60 times faster than prior art drive systems.

The z-axis control system 200 has five basic modes of operation: (1) abiased following (or height sensing) cutting mode, (2) a set-heightcutting mode, (3) a manual raise/lower mode, (4) a park mode, and (5) acalibration mode. The calibration mode is used to test the performanceof the z-axis control system 200 or to set up the system for the firsttime. In brief, the pressure within the air cylinders is varied until aneutral pressure is found. The neutral pressure is the pressure at whichthe cutting head 120 and the slide member 214 and other components(collectively referred to as “the axis”) will not move up or down withthe air brake released. The upper and lower limits of a neutral pressure“dead band” are found and recorded. Also, the upper and lower travellimits of the axis are found and recorded. These data are used to setthe values for the other movement modes, and the “dead band” data areused as a diagnostic tool to determine if the axis is in need ofservicing due to excessive friction.

FIG. 8 is a flowchart representation of a calibration routine 300 of thez-axis control system 200 in accordance with an embodiment of theinvention. First, the pressure within the air cylinders is set to adefault or neutral pressure 302 corresponding to a neutral, non-movingposition of the cutting head. Next, the air brake is released 304. Afterthe air brake is released, a determination is made whether the axis ismoving up 306. If the axis is moving up, the pressure within the aircylinders is incrementally decreased 308. The determination whether theaxis is moving up 306, and the action of decreasing the pressure 308,are repeated until the axis is no longer moving up.

If it is determined that the axis is not moving up 306, a determinationis made whether the axis is moving down 310. If z-axis is moving down,the pressure within the air cylinders is incrementally increased 312.The determination 310 and incremental pressure increase 312 are repeateduntil the axis is no longer moving down.

One may note that acts or actions 306 through 312 may not be necessaryto the calibration procedure 300 if the default pressure setting 302 isindeed a neutral pressure setting. If, however, the default pressuresetting 302 is not a neutral pressure setting, such as may be the casewhen, for example, one or more components of the cutting head have beenmodified or removed since the previous calibration, then the acts oractions 306 through 312 may be followed to establish an appropriateneutral pressure setting.

As shown in FIG. 8, if it is determined that the axis is not movingdown, another determination is made whether the axis is moving up 314.If it is determined that the axis is not moving up, the pressure isincrementally increased 316, and the calibration procedure 300 returnsto the determination whether the axis is moving up 314. Thedetermination 314 and the incremental pressure increase 316 are repeateduntil the axis is moving up.

If the axis is moving up 314, an upper threshold pressure is recorded bythe z-axis controller 318. The upper threshold pressure signifies thepressure in the air cylinders at which the axis will begin movingupwardly.

Next, a determination is made whether the axis is moving down 320. If itis determined that the axis is not moving down, the pressure isincrementally decreased 322. The calibration procedure 300 then returnsto the determination whether the axis is moving down 320. Thedetermination 320 and the incremental pressure decrease 322 are repeateduntil the axis is moving down.

If the axis is moving down 320, the z-axis controller records a lowerthreshold pressure 324. The lower threshold pressure signifies thepressure in the air cylinders at which the axis will begin movingdownwardly.

Next, the pressure in the air cylinders is increased to the upperthreshold pressure plus an incremental step pressure 326. Adetermination is then made whether the axis is moving 328. If the axisis moving, the speed of the upward movement of the axis is recorded 330.The determination whether the axis is moving 328 and about recording ofthe speed of upward movement 330 are repeated until the axis is nolonger moving, and has reached its upper limit of travel. If the axis isnot moving 328, an upper limit of travel is recorded 332.

The calibration procedure 300 then decreases the pressure in the aircylinders to the lower threshold pressure minus the incremental steppressure 334. Next, a determination is made whether the axis is moving336. If the axis is moving, the speed of the downward movement of theaxis is recorded 338. The determination 336 and the recording of thespeed of downward movement 338 are repeated until the axis is no longermoving, and has reached its lower limit of travel. If the axis is notmoving 336, a lower limit of travel is recorded 340. The calibrationprocedure 300 is then complete 342.

In the set-height cutting mode, the axis is moved manually orautomatically into place. When moved automatically into place, the axiswill move down until it engages the surface 14 of the workpiece 12 bylowering until the axis stops moving, then, if necessary, raising up tothe proper standoff distance. The z-axis control system 200 then assumesa neutral pressure with the air brake engaged.

In the manual raise/lower mode, the axis is raised or lowered ascommanded by the operator until the end of travel limits have beenreached, or until the wear ring 129 of the collision shield 127 contactsthe surface 14 of the workpiece 12. The axis may be raised or lowered,for example, by inputting a raise or lower movement command into thez-axis controller 190 by means of a keyboard (not shown). When thelimits of travel have been reached, all travel ceases. When a movementcommand is removed, or the end of travel is reached, the axis receives areverse-pressure signal to slow it down. The reverse-pressure signalmay, for example, be based on velocity of the axis. When the axis ismoving continuously, the axis seeks a constant velocity. Incrementalmoves may be based, for example, upon individual keystrokes of thekeyboard (or individual mouse clicks, etc.) that movie axis apredetermined distance either up or down. In either the incremental orcontinuous movement case, the movement is terminated by engaging the airbrake.

In the park mode, the axis is simply raised to its upper limit of traveland air brake is engaged. The pressure within the air cylinders is setat a neutral bias setting.

In the biased-following (or height-sensing) cutting mode, the axis has aslight downward bias pressure. The slight downward bias causes the axisto fall slowly, keeping the wear ring 129 in constant contact with thesurface 14 of the workpiece 12. Stiction in the up direction iscompensated for by rapidly moving the pressure up and down within thedead band between the lower threshold pressure and the upper thresholdpressure. The air brake 218 is not engaged.

FIG. 9 is a flowchart representation of a biased-following (orheight-sensing) routine 400 of the z-axis control system 200 inaccordance with an embodiment of the invention. In this embodiment, thebiased-following routine 400 begins by decreasing the pressure in theair cylinders to the lower threshold pressure minus an incremental steppressure 402. Next, the air brake is released 404. A determination isthen made whether the axis is moving 406. If the axis is moving, thedetermination 406 is repeated indefinitely until the axis is not moving.If the axis is not moving, the pressure in the air cylinders is variedbetween the upper and lower threshold pressures 408. Next, adetermination is made whether a collision has occurred 410. If acollision has not occurred, the collision determination 410 is simplyrepeated indefinitely. If a collision has occurred, the z-axis controlsystem 200 is halted 412. Alternately, if a collision has occurred, thepressure in the air cylinders may be increased to rapidly raise the axisaway from the workpiece.

Another advantage of the z-axis control system 200 is that itautomatically compensates for changes in friction and/or weight ofsystem components such as, for example, the air cylinders 208, thelinear rail 216, the guide blocks 206, wear parts such as bearings, andother system components. The z-axis controller 230 automaticallycompensates by adjusting the pressure within the air cylinders 208 tolower the slide member 214, maintaining the engagement of the wear ring129 with the surface 14 of the workpiece 12 in the biased-following modeof operation, or at a constant height above the surface 14 in theset-height mode of operation. In this way, the standoff distance d ismaintained at the desired value despite changes in friction and/orweight of the various system components.

Improved apparatus and methods for z-axis control and collision recoveryof cutting heads of waterjet cutting systems have been shown anddescribed. From the foregoing, it will be appreciated that althoughembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit of the invention. Thus, the present invention is not limitedto the embodiments described herein, but rather is defined by theclaims, which follow.

What is claim is:
 1. A method of collision detection and recovery for acutting head assembly of a waterjet cutting system, comprising:disengageably mounting the cutting head assembly on a mounting assemblyhaving a collision-sensing circuit that senses an electrical contactbetween the cutting head assembly and the mounting assembly; monitoringa collision-detection signal status from the collision-sensing circuit;colliding the cutting head with a foreign object; disengaging thecutting head assembly from the mounting assembly to break the electricalcontact and change the collision-detection signal status; determiningthat a collision has occurred from the change in the collision-detectionsignal status; and transmitting a stop signal to the waterjet cuttingsystem.
 2. The method according to claim 1, further comprisingdisengaging the cutting head from the foreign object.
 3. The methodaccording to claim 1, further comprising transmitting an actuationsignal to a z-axis control system attached to the cutting head.
 4. Themethod according to claim 1, further comprising re-engaging thedisengageable mounting assembly.
 5. The method according to claim 1wherein transmitting a stop signal to the waterjet cutting systemcomprises transmitting a shutoff signal to a high-pressure fluid sourceof the waterjet cutting system.
 6. A method of controlling a height of acutting head of a waterjet cutting system over a surface of a workpiece,comprising: rigidly coupling a first end of a substantially rigidcontact member to the cutting head; moving the cutting head toward theworkpiece until a second end of the substantially rigid contact memberexerts a contact force on the workpiece; exerting an adjustablycontrollable support force on the cutting system; adjusting the supportforce until the support force is substantially equal in magnitude andsubstantially opposite in direction to the contact force; and slightlyreducing the support force to slightly bias the contact member intoengagement with the workpiece.
 7. The method according to claim 6wherein providing an adjustably controllable support force includesadjustably pressurizing an air cylinder coupled to the cutting head. 8.The method according to claim 6 wherein slightly reducing the supportforce to slightly downwardly bias the contact member includes slightlyreducing an air pressure in an air cylinder coupled to the cutting head.9. The method according to claim 6, further comprising: monitoring acollision detection signal; and transmitting a control signal inresponse to the collision detection signal.
 10. The method according toclaim 9 wherein transmitting a control signal in response to thecollision detection signal includes transmitting a stop signal to acontrol gantry of the waterjet et cutting system.
 11. The methodaccording to claim 9 wherein transmitting a control signal in responseto the collision detection signal includes transmitting an controlsignal to an airflow control valve to increase an air pressure in an aircylinder coupled to the cutting head.
 12. The method according to claim6, further comprising cyclically adjusting the adjustable support forcewithout changing the height of the cutting head.
 13. The method of claim4 wherein the collision-detection signal status is the absence of anysignal when the cutting head assembly is engaged with the mountingassembly, and wherein the change in the collision-detection signalstatus is a change to the existence of a signal.